The invention relates to a method and apparatus for setting registration in a multicolor printing machine having a number of exposure devices for the digital production of color separations, the print results being registered and being used to control the production of the color separations such that accurate in-register prints are achieved, and a change in the dimensions of the printing substrates being determined by the print, in order to take this change into account for renewed printing.
Printing colored illustrations, in particular colored images, is carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan as well as black. If required, special colors are added. By overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the accurate in-register overprinting of the color separations. In the case of digital printing processes, for example electrostatic printing processes, the maintenance of the register of the overprinting is achieved by the image production devices being controlled in such a way that the color separations meet one another in-register when they are transferred to a printing substrate.
A method and apparatus for color printing are disclosed by EP 0 469 282 A2. In the subject matter of this prior art, these are used to coordinate color separations of a print which are fused separately, that is to say joined to the paper by being melted on, such that the next color separation always fits in exact registration on the color separation or separations already fused. However, in the case of many multicolor printing machines, this problem is dispensed with by all the color separations being fused together.
However, the change in their size has to be taken into account, instead it concerns the fact that a printing substrate is to be printed on both sides, in recto and verso printing for a clean appearance of the prints, it is necessary for the recto printing and the verso printing to be located so that they coincide on the printing substrate. Otherwise the image area of one print, which did not coincide, with the other print would show through the printing substrate. Since the recto printing has to be completed before the verso printing is performed, the problem also occurs here that, for example, as a result of the fusing, the substrate size and, therefore, in a corresponding way the image size changes. If, therefore, recto and verso prints were of equal size when transferred to the printing substrate, it would not be possible for coincidence to be achieved, because of the dimensional changes.
The invention is based on the object of configuring a method and apparatus such that coincidence of the recto and verso prints can be achieved. With regard to the method, the object is achieved in that the changes in the dimensions of the recto print from the transfer to the printing substrate up to the completion of the recto print are determined and are used to control the digital production of the color separations for the verso print such that, after being printed, it coincides with the recto print. With regard to the apparatus, the object is achieved by the at least one sensor being such that the changes in the dimensions of the recto print from the transfer to the printing substrate up to the completion of the recto print can be determined, and that the controller is designed such that it uses the changes in the dimensions to control the digital production of the color separations for the verso print such that, after being printed, it coincides with the recto print.
With regard to the apparatus, the object is achieved by the at least one sensor being designed and arranged in such a way that the changes in the dimensions of the recto print from the transfer to the printing substrate up to the completion of the recto print can be determined, and that the controller is designed in such a way that it uses the changes in the dimensions to control the digital production of the color separations for the verso print in such a way that, after being printed, it coincides with the recto print.
The invention makes it possible to achieve exact coincidence of recto and verso prints, it being immaterial whether the dimensional changes have been caused by the fusing or in another way. As a result, for example for color copiers, it is possible to achieve a quality in the area of recto and verso printing which previously could not be attained. Since a registration control system is present in any case for the digital production of the color separations, the measure according to the invention often merely requires the appropriate setting up of the existing controllers. In addition, sensors, which are present, in any case can often be used as sensors for registering the dimensions of the recto print. It is therefore often possible, by the invention, to achieve a significant improvement in quality without great outlay and therefore very economically.
Determining the dimensional changes of the recto print from the transfer to the printing substrate up to the completion of the recto print can be performed in various ways. For example, it is possible to use values from image files or values based on experience, or to measure the dimensions of the image area directly or to determine them in another way, but it is also possible for the dimensions of the printing substrate up to the printing and after the completion of the recto print to be used as a correlating measure of the changes in the dimensions of the recto print.
One configuration of the apparatus provides for at least one sensor to be arranged such that the changes in the dimensions of the recto print from the transfer to the printing substrate up to the completion of the recto print can be determined. Here, provision can also be made for at least one sensor to be arranged such that the dimensions of the printing substrate up to the printing and after the completion of the recto print can be registered as a correlating measure of the changes in the dimensions of the recto print.
An advantageous configuration provides for the dimensions of the recto print to be determined after fusing and compared with the dimensions of the digitally produced color separations of the recto print, the changes in the dimensions being used to control the digital production of the color separations of the verso print. The advantage is that the data relating to the dimensions of the digitally produced color separations of the recto print are present in any case, and it is, therefore, only additionally necessary to determine the dimensions of the completed recto print. Since fusing is the operation during which the changes in dimensions essentially take place, it is expedient to determine the dimensions of the recto print after fusing. With respect to the apparatus, provision is then made for the at least one sensor to be arranged such that it determines the dimensions of the recto print after fusing, and for the controller to compare these dimensions with the dimensions of the digitally produced color separations of the recto print, and controls the digital production of the color separations of the verso print in order to achieve coincidence. The dimensions of the digitally produced color separations can be taken from image files, which are stored in a memory. Such a memory with image files for controlling the production of the prints is present in any case on a machine, and it is, therefore, the simplest solution to fall back on these image files.
In order to register the changes in the dimensions, both the widths of the recto print and the lengths, or both dimensions can be registered. With respect to the apparatus, provision is then made for at least one sensor for registering these dimensions to be arranged appropriately. If only one of the dimensions is registered, it is then possible for the other dimension to be calculated from a previously determined relationship between the length and width changes. With respect to the apparatus, the controller must then appropriately perform this calculation.
With respect to the production of the color separations of the verso print, the changes in the dimensions are best taken into account by the line spacing of the image lines and/or the line length of the color separations of the verso print being changed in order to achieve coincidence. In this case, the verso print is given exactly the format of the recto print on the other side of the printing substrate in order to achieve coincidence, which is then maintained during the fusing of the verso print as well. With respect to the apparatus, the controller changes the line spacings of the image lines and, if appropriate, also the line length appropriately.
Since, in a normal machine configuration, a printing substrate is turned after the recto printing and has to be fed to the printing units again, an expedient configuration provides that, before the printing substrate is fed in again, a measurement of the position of the recto print is made, in order also to take into account any shift in the position of the recto print for controlling the production of the color separations of the verso print in order to achieve coincidence. This ensures that coincidence can be achieved in spite of turning and re-feeding, even when a shift in position, for example, an error in the angular alignment of the printing substrate, has occurred.
With respect to the apparatus, provision is then made for at least one sensor to be arranged to measure the position of the recto print as a printing substrate is fed back to the printing units, and for the controller to also take into account a shift in the position of the recto print to control the production of the color separations of the verso print in order to achieve coincidence. For this purpose, the position of a printing substrate can be registered at the end of the transport belt and/or at the start of said transport belt, after being turned and fed in again. In order to have registration of the position in this way, it is also possible to use sensors, which are present in any case on a printing machine, such as the sensors for aligning the printing substrates.
The invention will be explained below using the drawings, in which:
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.